John,
Thanks. This is a very interesting aspect which again needs a thread of its own - STOVL aerodynamic derivatives are a very different field!
1hr slots,
The Dutch roll is a dynamic stability root of the lateral-directional equation of motion. It is an oscillatory motion which, therefore, can be defined by a frequency and a damping ratio. If it has greater than critical damping, it will appear as an aperiodic convergent motion, but can still be defined by the same parameters. So, you must define what you mean by "Dutch roll stability". I suspect that you mean "damping" in terms of either the number of cycles or time to damp. If so, an increase in directional static stability actually reduces the Dutch roll damping ratio (whilst increasing frequency and thus giving a similar time to damp). However, as the Dutch roll is triggered by sideslip, an increase in directional static stability will suppress sideslip development and may prevent the Dutch roll being excited although the actual damping ratio of the open loop response will have been reduced. The effect of anhedral (which reduces lateral static stability or increases lateral static instability) on the Dutch roll frequency and damping is not that great. The largest effect that it has is on the roll:yaw ratio and the phase relationship between roll rate and yaw rate. Strong lateral stability produces a high roll:yaw ratio (wing rock), weak lateral stability produces a low roll:yaw ratio (snaking). Lateral static instability may produce an out of phase motion (rolling left whilst yawing right). Adding anhedral to a wing would not be a normal way for a designer to improve Dutch roll damping! Overall, always be careful to define what you mean by "stable/unstable"; they are ambiguous words.